As electronic industry continues to advance, electronic components such as central processing units (CPUs), are made to provide faster operational speeds and greater functional capabilities. When a CPU operates at a high speed, its temperature frequently increases greatly. It is desirable to dissipate the heat generated by the CPU quickly. To solve this problem of heat generated by the CPU, a cooling device is often used to be mounted on top of the CPU to dissipate heat generated thereby.
A conventional cooling device comprises a heat sink with a base for contacting with the CPU to absorb heat therefrom and a fan mounted on the heat sink for generating forced airflow to the heat sink to thereby enhance the heat dissipation capability of the heat sink. However, as the CPU operates more and more fast and therefore generates larger and larger heat the conventional heat sink, which transfers heat via heat conduction means, cannot meet the increased heat dissipating requirement of the CPU. Subsequently, heat pipes have been developed to be incorporated into the heat sink for improving heat dissipation capability of the heat sink. However, the improving effect of the heat pipe is limited due to the CPU has a small heat dissipation surface which limits the number of the used heat pipe. Furthermore, some heat pipes may not work normally in inclined state in which the capillary action of capillary structure arranged inside the heat pipe is affected by the gravity.
It is well known that the heat transfer efficiency by phase change of fluid (i.e. from liquid to vapor) is better than other mechanisms, such as heat convection and heat conduction. It is also well known that heat absorbed by fluid having a phase change is ten times more than that the fluid does not have a phase change. Accordingly, recently, cooling devices with boiling chambers have been developed. FIGS. 13-14 show a conventional boiling chamber cooling device which includes a boiling chamber 10′ for contacting with a heat generating component 20 and a heat sink 30′ mounted on the boiling chamber 10′. The boiling chamber 10′ contains therein working fluid 15′ which boils when absorbs heat from the heat generating component 20 and transfers the heat to the heat sink 30′ via the boiling fluid 15′ contacting with a top wall of the chamber 10′. However, in this kind of conventional boiling chamber cooling device, a predetermined space exists between the top wall of the chamber 10′ and the top surface 15a′ of the fluid 15′ so that the fluid 15′ must boil and therefore be expanded before it contacts with the top wall of the chamber 10′ in order to transfer the absorbed heat to the heat sink 30′. A heat transfer threshold thus exists in the boiling chamber 10′ since the predetermined space existing between the top wall of the chamber 10′ and the fluid 15′ results in the fluid 15′ not capabling of transferring heat from the heat generating component 20 to the heat sink 30′ before the fluid 15′ boils. Furthermore, the fluid 15′ generates a locally intensive agitation at an area located just above the heat generating component 20 since it is impossible that heat generated by the heat generating component 20 is spread to the whole bottom surface of the chamber 10′ without any delay, which results in a wave formation at the top surface 15a′ of the fluid 15′ when the fluid 15′ boils. Thus, the boiling fluid may not continuously and stably thermally contact with the top wall of the chamber 10′. A detailed description about density-controlling instability is disclosed in the article TRANSPORT PROCESSES IN BOLING AND TWO-PHASE SYSTEMS, HEMISPHERE PUBLISHING CORPORATION, 1976, Washington, Chapter 2 and Section 9.2.2.2, which is incorporated herein by reference. The heat transfer effect of the boiling chamber 10′ is therefore reduced. Moreover, the predetermined space existing between the top wall of the chamber 10′ and the top surface 15a′ of the fluid 15′ results in the level of the fluid 15′ may be lower than that of the heat generating component 20 when the chamber 10′ locates at inclined state, which results in the heat transfer effect of the boiling chamber 10′ being significantly reduced.
For the foregoing reasons, therefore, there is a need in the art for a cooling device which overcomes the above-mentioned problems.